Nitro‐Carbazole Based Oxime Esters as Dual Photo/Thermal Initiators for 3D Printing and Composite Preparation

A series of Type I photoinitiators (PIs) based on a nitrocarbazole scaffold are developed and examined for the first time as photoinitiators for visible light photopolymerization. Three oxime esters (OXE‐M, OXE‐V, OXE‐P) varying by the terminal groups (acetyl, acryloyl and benzoyl) attached via the oxime ester group are originally prepared. As a result of this, the three PIs exhibit excellent photoinitiation abilities in the presence of acrylate monomers upon LED@ 405 nm irradiation. Markedly, OXE‐M exhibits a better performance than the benchmark Type I phosphine‐oxide (diphenyl(2,4,6‐trimethylbenzoyl)phosphine oxide TPO). Chemical mechanisms supporting the polymerization process with these PIs are investigated by steady state photolysis, molecular orbital calculations and real‐time Fourier transformed infrared spectroscopy. After the cleavage of N─O bond and decarboxylation, free radicals are generated to initiate the free radical polymerization efficiently. Free radical photopolymerization of OXE‐M is applied in direct laser write and 3D printing. Interestingly, OXE‐M exhibits thermal initiation behaviors in monomers and can be used as dual photo and thermal initiators. The highly opaque feature of carbon fibers makes it difficult for light penetration, so dual photo/thermal curing are used here to prepare carbon fiber composites. Thermal and photochemical initiators correspond to two different families and very few structures are able to initiate both thermal and photochemical polymerization processes. In this work, a new family of dual photo and thermal initiators is proposed. This can be highly worthwhile to combine the advantages of both polymerization processes. Examples in 3D printing and composites preparation are provided.